The present invention relates to a scanning probe microscope capable of measuring specimen surface information with atomic-order resolution, and more particularly, to a scanner system and a piezoelectric micro-inching mechanism used in a scanning probe microscope.
Scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs) are typical examples of scanning probe microscopes (SPMs).
In the scanning tunneling microscopes, which are the original version of scanning probe microscopes, the surface shape of a specimen is measured with atomic-order resolution by utilizing a tunneling current flowing between a metallic probe and an electrically conductive specimen which are located close to each other. Utilizing the tunneling current, the scanning tunneling microscopes is used to observe electrically conductive specimens only.
The atomic force microscopes have been developed by utilizing the servo technique and other STM techniques. In these microscopes, the surface shape of a specimen is measured with atomic-order resolution by utilizing an atomic force which acts between atoms in the apex of a probe and the surface of a specimen. Accordingly, the atomic force microscopes is used to observe electrically insulating specimens as well as conductive ones.
In order to enjoy high resolution, the scanning probe microscopes require a scanning mechanism that can control the relative positions of the probe and the specimen with high accuracy. In general, these microscopes use a piezoelectric micro-inching mechanism, especially a cylindrical piezoelectric scanner or so-called tube scanner.
FIGS. 11A, 11B and 11C show an arrangement of one such tube scanner. FIG. 11A is a perspective view of the tube scanner, FIG. 11B is a development showing outside electrodes of the scanner, and FIG. 11C is a development showing an inside electrode.
A tube scanner 1040 comprises a piezoelectric ceramic 1041 in the form of a hollow cylinder, Z-axis driving electrode 1044, X-axis driving electrodes 1042a and 1042b, Y-axis driving electrodes 1043a and 1043b, and common electrode 1045. The electrodes 1044, 1042a, 1042b, 1043a and 1043b are arranged outside the ceramic 1041, and the electrode 1045 inside. The X-axis driving electrodes 1042a and 1042b are located in positions at an angular distance of 90xc2x0 from the Y-axis driving electrodes 1043a and 1043b, respectively. The Z-axis driving electrode 1044, X-axis driving electrodes 1042a and 1042b, and Y-axis. driving electrodes 1043a and 1043b face the common electrode 1045 across the piezoelectric ceramic 1045.
The tube scanner 1040 is displaced in the X-axis direction as opposite-polarity voltages are applied to the X-axis driving electrodes 1042a and 1042b, individually, with the common electrode 1045 grounded, and is displaced in the Y-axis direction as opposite-polarity voltages are applied to the Y-axis driving electrodes 1043a and 1043b, individually. The piezoelectric ceramic 1041 extends or contracts to be displaced in the Z-axis direction, depending on the polarity of a voltage applied to the Z-axis driving electrode 1044.
In order to obtain a substantial displacement in the Z-axis direction in the tube scanner shown in FIGS. 11A to 11C, the height of the Z-axis driving electrode 1044 or the voltage applied thereto must be increased.
If the height of the Z-axis driving electrode 1044 is increased, however, the tube scanner 104 is lengthened inevitably, resulting in lowered resonance frequency and hence poorer responsivity.
An expensive power source and a driver circuit are needed to apply a high voltage to the Z-axis driving electrode 1044 to drive it.
An example of a scanning probe microscope which uses a tube scanner is described in Jpn. Pat. Appln. KOKAI Publication No. 5-312564. FIG. 12 shows a configuration of this microscope.
As shown in FIG. 12, a specimen 1103 is fixed to a free end of a cylindrical piezoelectric element (tube scanner) 1102. The piezoelectric element 1102 is moved for scanning in the X- and Y-directions in response to X- and Y-scan signals which are generated from an X-scan signal generator 1111 and a Y-scan signal generator 1112, respectively. Thus, the specimen 1103 on the piezoelectric element 1102 is scanned in the X- and Y-directions. The cantilever 1104, which is located close to the specimen 1103, is deflected in the Z-direction, depending on the surface irregularity of the specimen 1103.
Light emitted from a laser diode 1105 is reflected by a mirror 1106, to be incident upon the surface of the cantilever 1104, and the reflected light is projected on a photodiode 1107. The deflection of the cantilever 1104 is detected by monitoring the position of a beam spot on the photodiode 1107 by means of a differential amplifier 1108. The cylindrical piezoelectric element 1102 is feedback-controlled in the Z-direction by means of a servo circuit 1109 to keep the deflection constant.
If a computer 1110 directly fetches as height information or surface irregularity information a voltage applied to the piezoelectric element to drive it in the Z-direction, an obtained image is subject to distortion attributable to hysteresis, creeping, etc., which are peculiar to the piezoelectric element.
Accordingly, an optical fiber 1114 is located inside the cylindrical piezoelectric element 1102, and the Z-direction displacement of a mirror 1116 is detected through the fiber 1114 by means of optical interferometer 1115. The computer 1110 fetches the resulting displacement signal as height information or surface irregularity information, thereby forming a surface irregularity image.
Referring now to FIG. 13, there will be described a drawback of the system shown in FIG. 12. If the cylindrical piezoelectric element 1102 is displaced in the X-direction, as shown in FIG. 13, the mirror 1116 located on the undersurface of a specimen stage 1120 which is fixed to the free end of the element 1102, is displaced at an angle to the end face of the optical fiber 1114 in the X-direction.
Accordingly, an object of measurement by means of the optical interferometer 1115 shifts its position from a point a on the mirror 1116 to another point b. Therefore, a Z-displacement measured by the interferometer 1115 involves the influence of change of the inclination of the mirror 1116 or surface conditions. Thus, the shifts of the inclination of the object of measurement and the measuring position, which are caused as the tube scanner is displaced in the X-direction, are primary factors that lower the accuracy of the Z-displacement measurement.
A scanning probe microscope in which an optical microscope is incorporated in an atomic force microscope is described in Jpn. Pat. Appln. KOKAI Publication No. 8-285865. As shown in FIG. 14, for example, this microscope comprises a quadruple scanner 1206 having a quadruple electrode, a cantilever displacement sensor 1208 in the form of an optical lever, and an objective lens 1210 for optical microscope. The scanner 1206 moves a cantilever 1202 with respect to a specimen 1204. The sensor 1208 optically detects the deflective displacement of a free end of the cantilever 1202, which is based on the interaction, e.g., atomic force or frictional force or adsorptive force or contact force, between the apex of a probe 1212 and the specimen 1204. The objective lens 1210 is inserted in the scanner 1206 so that a scanning region for the cantilever 1202, i.e., the surface of the specimen 1204, can be optically observed through the lens 1210.
After the probe 1212 on the cantilever 1202 is situated in the scanning region with use of the objective lens 1210, the scanner is 1206 is feedback-controlled in the Z-direction in response to a Z-displacement signal from the displacement sensor 1208. At the same time, the distance between the apex of the probe 1212 and the surface of the specimen 1204 is kept constant as the probe 1212 is moved for scanning in the X- and Y-directions. Thus, specimen information based on the aforesaid interaction is detected and displayed.
In this scanning probe microscope, both the displacement sensor 1208 and the cantilever 1202 are mounted on the end of the quadruple scanner 1206, so that the resonance frequency of the scanner 1206 is not very high. Inevitably, therefore, the scanning speed of the scanner 1206 is so low that the measuring time cannot be shortened. Since the rigidity of the scanner 1206 is low, moreover, measurement data are susceptible to vibration and other disturbance noises.
An object of the present invention is to provide a piezoelectric micro-inching mechanism capable of enlarging displacement in the Z-axis direction without requiring use of high driving voltage or lowering its resonance frequency.
In order to achieve the above object, a piezoelectric micro-inching mechanism according to a first aspect of the invention comprises: a first piezoelectric drive section displaceable along a first axis and a second axis; and a second piezoelectric drive section displaceable along a third axis perpendicular to the first and second axes, the first and second piezoelectric drive sections being formed integrally with each other, the first piezoelectric drive section including a cylindrical piezoelectric element, a common electrode internally surrounding the inside of the piezoelectric element, and four driving electrodes arranged at intervals in the circumferential direction outside the piezoelectric element, the second piezoelectric drive section including a cylindrical piezoelectric element, an inside electrode internally surrounding the inside of the piezoelectric element, and an outside electrode surrounding the outside of the piezoelectric element.
Here xe2x80x9cthe first and second piezoelectric drive sections being formed integrally with each otherxe2x80x9d implies both a structure that includes the first and second piezoelectric drive sections comprising the common electrode, driving electrodes, and inside and outside electrodes provided for the one cylindrical piezoelectric element and a structure obtained by connecting the first and second piezoelectric drive sections that are formed separately from each other.
The first and second piezoelectric drive sections are connected by bonding them directly or by means of a connecting member or by means of an elastic member as well as the connecting member.
Normally, the common electrode of the first piezoelectric drive section is grounded, and opposite-polarity voltages are applied individually to two opposite driving electrodes so that the first piezoelectric drive section is displaced along the first or second axis. Preferably, the second piezoelectric drive section is displaced along the third axis as opposite-polarity voltages are applied individually to its inside and outside electrodes. This displacement doubles the displacement that is obtained with the inside electrode grounded.
Another object of the invention is to provide a scanner system for a scanning probe microscope capable of accurately measuring the Z-displacement of an object of scanning.
In order to achieve the above object, a scanner system according to a second aspect of the invention comprises: a holding section for holding an object of scanning; a Z-displacement generator for applying a Z-direction displacement to the holding section; an XY-displacement generator for applying X- and Y-direction displacements to the holding section; a connecting member for connecting the Z-displacement generator and the XY-displacement generator; and a Z-displacement detector for detecting the Z-direction displacement of the holding section. The holding section is provided directly on the Z-displacement generator, while the Z-displacement detector is attached to the connecting member.
The XY-displacement generator comprises a cylindrical piezoelectric scanner including a cylindrical piezoelectric element, a common electrode internally surrounding the inner peripheral surface of the piezoelectric element, and four driving electrodes arranged at intervals in the circumferential direction of the outer peripheral surface of the piezoelectric element, for example. The Z-displacement generator comprises a cylindrical piezoelectric actuator including a cylindrical piezoelectric element, an inside electrode internally surrounding the inner peripheral surface of the piezoelectric element, and an outside electrode surrounding the outer peripheral surface of the piezoelectric element.
Further, the Z-displacement detector is situated inside the cylindrical piezoelectric actuator.
Still another object of the invention is to provide a scanner system for a scanning probe microscope, enjoying high resonance frequency and high rigidity.
In order to achieve the above object, a scanner system according to a third aspect of the invention comprises: a scanning mechanism for moving a probe on a cantilever in horizontal directions along the surface of a specimen to be scanned; a moving mechanism for moving the probe on the cantilever in a vertical direction with respect to the specimen; and a displacement detecting mechanism situated between the scanning mechanism and the moving mechanism and capable of detecting the displacement of the cantilever.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.